Cardiac stimulation devices deliver appropriately timed electrical stimulation pulses to a patient's heart to maintain a normal heart rhythm or improve synchronization of heart chambers. Patients having bradycardia, abnormalities of the heart's natural conduction system, a propensity for arrhythmias, cardiac-related breathing disorders, hemodynamic insufficiency, or other cardiac-related conditions may benefit from cardiac pacing therapies delivered in one or more heart chambers.
In order to effectively pace the heart, an electrical impulse delivered to the heart must have sufficient energy to depolarize the myocardial cells. Depolarization of the myocardial cells in response to a pacing pulse is often referred to as “capture.” The cardiac electrogram signal evidencing capture, which is a P-wave in the atria or an R-wave in the ventricles, is generally referred to as an “evoked response.” The lowest pacing pulse energy that captures the heart may be referred to as the “pacing threshold” or “capture threshold”. The amplitude and duration of a pacing pulse are preferably set to produce a pacing pulse energy somewhat greater than the pacing threshold in order to ensure effective cardiac pacing. However, in order to prolong the battery life of the implanted cardiac stimulation device, it is desirable to program the pacing pulse energy to be a minimum value that is considered safely above the pacing threshold.
Pacing threshold, however, can change over time due to tissue encapsulation of the pacing electrodes, lead movement, changes in the patient's clinical condition, changes in medical therapy, or other causes. Therefore, monitoring or detecting pacing threshold changes has diagnostic value as well as therapy delivery implications. A rise in pacing threshold can result in loss of capture and ineffective pacing therapy. Modern pacemakers typically include automatic pacing threshold search algorithms that automatically adjust the pacing pulse energy to ensure pacing pulses remain above the pacing threshold, even if it varies over time. A pacing threshold search may deliver pacing pulses starting at an initially high pulse energy that is greater than the pacing threshold and then progressively decrease the pulse energy until capture is lost. The lowest pulse energy at which capture still occurs is determined as the pacing threshold. In order to reliably determine a pacing threshold, the cardiac pacing device must reliably discriminate between capture and loss of capture.
Capture management is important in the maintenance of effective cardiac pacing. If loss of capture is not recognized by the pacing system, prolonged episodes of subthreshold cardiac pacing may result, during which pacing is ineffective in maintaining a base heart rate, responding to needed increases in heart rate or treating a cardiac-related condition. Pacing-dependent patients may become symptomatic, requiring clinical care and even hospitalization.
One method that has been implemented in commercially available devices for detecting capture is to sense the evoked response following a pacing pulse. Evoked response sensing may be used to verify capture during pacing threshold searches and during normal cardiac pacing operations to ensure that effective pacing is provided. If a loss of capture is detected, as evidenced by the absence of an evoked response following a pacing pulse, a back-up pacing pulse of higher energy may be delivered and a pacing threshold search may be triggered to reset the pacing pulse energy. Pacing systems may provide continuous ventricular capture management by searching for an evoked response following each ventricular pacing pulse. Other systems may provide periodic ventricular capture management by periodically checking for an evoked R-wave or periodically performing a pacing threshold search. Continuous capture management is perceived to be superior to periodic capture verification due to the importance of capture management in maintaining effective pacing therapies.
Sensing of an evoked P-wave for verification of atrial capture, on a periodic or continuous basis, however, is generally more difficult than evoked R-wave sensing because the amplitude of the P-wave is considerably less than the amplitude of the R-wave. Polarization at the electrode-tissue interface causes an afterpotential signal that can saturate sense amplifiers included in the cardiac pacing device and mask an evoked response signal. Typically, a blanking interval is applied to sense amplifiers during and immediately following a pacing pulse to prevent saturation of the amplifiers. The polarization artifact may diminish during the blanking interval, however, it may still interfere with evoked response sensing. Low-polarization electrodes have been proposed for reducing the polarization artifact. See for example U.S. Pat. No. 4,502,492, issued to Bornzin, or U.S. Pat. No. 6,430,448, issued to Chitre, et al.
Various alternative approaches to direct evoked response sensing have been proposed for overcoming limitations due to pacing polarization artifact. Selection of separate sensing electrodes for sensing the evoked response, different than the electrode pair used for delivering the pacing pulse, can reduce or eliminate polarization artifact problems. Sensing a far-field signal related to an evoked response, as opposed to the near-field evoked response signal, or sensing a conducted depolarization away from the pacing site has also been proposed. See for example, U.S. Pat. No. 5,324,310 issued to Greeninger, U.S. Pat. No. 5,222,493 issued to Sholder, U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 6,434,428 issued to Sloman, et al., and U.S. Pat. App. No. 20010049543, issued to Kroll. Depending on the associated lead system in use, however, alternative evoked response sensing electrodes may not always be available. With regard to atrial capture management, far-field P-waves may still be difficult to detect because of their relatively low amplitude. Detection of conducted depolarizations as evidence of capture at a pacing site is dependent on intact myocardial conduction pathways.
A method for periodically performing an atrial pacing threshold test is disclosed in U.S. Pat. No. 5,601,615 issued to Markowitz et al, incorporated herein by reference in its entirety. The atrial threshold test regime in patients having intact A-V conduction detects atrial loss of capture by the absence of a detected ventricular depolarization. In a second algorithm, a premature atrial pace is delivered, and the presence of an atrial event at the end of a measured sinus escape interval is declared to be an atrial loss of capture. Other methods for determining an atrial capture threshold are generally disclosed in U.S. Pat. No. 6,263,244 issued to Mann et al., U.S. Pat. No. 6,259,950 issued to Mann et al., U.S. Pat. No. 6,243,606 issued to Mann et al., and U.S. Pat. No. 6,295,471 issued to Bornzin et al.
It is apparent from the above discussion, however, that it is desirable to provide atrial capture management that is continuous rather than relying on periodic pacing threshold searches. Because of the various limitations described above, a cardiac pacing system capable of providing continuous atrial capture management is not presently available. It is desirable, therefore, to provide a cardiac stimulation system capable of effectively monitoring for atrial and/or ventricular capture on a continuous basis.